Generally, a turbocharger is configured to increase efficiency of an engine or a gas turbine of a vehicle. One of the most important factors in designing a turbocharger with excellent reliability and high efficiency is in the selection of a suitable bearing while considering driving conditions. The bearing is a vital mechanical component which significantly influences the efficiency and dynamic stability of the turbocharger. Thus, some problems relating to turbocharger systems are directly or indirectly related to the types of bearings used.
Turbochargers are commonly developed for diesel vehicles since diesel vehicles are more common.
Hereinafter, A prior art turbochager will be described with a reference to FIG. 1.
FIG. 1 is a cross sectional view showing a prior art turbocharger. A rotating body of a conventional turbocharger is supported by a ball bearing. However, if the ball bearing supports the rotating shaft of the turbocharger at high speeds and high temperature conditions, this may result in heavy friction loses causing structural thermal deformation or fractures. Thus, the prior art turbocharger illustrated in FIG. 1 proposes the use of an airfoil bearing. A rotating shaft (20) is built in a housing (10) while a turbine (1) and an impeller (2) is attached to both ends of the rotating shaft (20) respectively. A sealing member (30) is located between the shaft and the impeller (2) which prevents leakage from the shaft in an axial direction. An airfoil journal bearing (40) is located between the rotating shaft (20) and the housing (10) in order to produce dynamic-pressure which causes rotating shaft (20) to float. A thrust pad (50) is located in the middle portion of the rotating shaft (20) in order to axially support the rotating shaft (20). An airfoil thrust bearing (80) is mounted to the housing (10) on both sides of the thrust pad (50). An air-cooled cooler (60) is provided to the housing (10) in order to cool the rotating shaft (20), the airfoil journal bearing (40) and the thrust pad (50).
The rotating shaft (20), thrust pad (50) and the sealing member (30) are integrated into one body through a grinding process. In order to obtain sufficient dynamic pressure, the gap between the rotating shaft (20) and the airfoil journal bearing (40) should be minimized. Thus, the rotating shaft (20) should be sufficiently thick throughout the entire corresponding housing (10).
In order to improve thermal endurance, integrated rotating body (20), thrust pad (50) and sealing member (30) go through a solid-lubricant coating process, an abrasion process and a surface polishing process. The solid lubricant process produces less friction.
Heat-proof processing becomes a more important factor when the turbocharger operates in an environment with higher temperature and higher speed. However, if components of the turbocharger such as the rotating shaft (20), the thrust pad (5), the sealing member, etc. are integrated into a single body as in the prior art turbocharger, it is difficult to coat each of the components with the solid-lubricant uniformly. The corner portions become especially difficult to coat. Furthermore, components may become fractured during the abrasion process and/or the polishing process. Thus, the success rate of the heat-proof process (solid-lubricant coating, abrasion, polishing) is no more than about 20%.
Also, as mentioned above, the thickness of a journal portion (21) of the rotating shaft (20) should be heavy enough so that the airfoil journal bearing (40) can form sufficient dynamic pressure. Thus, material may be wasted. In addition, the inclusion of a heavy journal portion may also cause a very large moment of inertia causing a detrimental turbo-lag condition during rapid acceleration.
Moreover, there is another problem in that it is difficult to maintain cooling efficiency with the air-cooled cooling method in extreme environments.